Surface active compounds, or surfactants, are employed in a variety of processes for the purpose of reducing surface tension and distributing immiscible phases or compounds amongst each other as, for example, emulsions or dispersions, or for separating phases from distributed systems. The various surfactants used include a class of surface active compounds which can be employed to distribute clathrate hydrates of natural gases (hereinafter referred to as gas hydrates) throughout produced well fluids in order to prevent the plugging of pipelines and other conduits by these hydrates. The chemical, physical, and thermodynamic properties of these hydrates are described in detail in E. D. Sloan's book “Clathrate Hydrates of Natural Gases”, Marcel Dekker, NY, 1998.
Low boiling hydrocarbons, such a methane, ethane, propane, and isobutane, as well as carbon dioxide and hydrogen sulfide are present in the fluids produced from natural gas and crude oil recovery wells and are capable of forming gas hydrates with water which is also typically present in the fluids produced from natural gas and crude oil recovery wells. The mixture of water and the described well fluid components will form hydrates at the right conditions of high pressure and low temperature. For practical purposes gas hydrates are found particularly in deep sea hydrocarbon producing wells where low temperatures and high pressures are common. The hydrates have a tendency to occlude or block these pipelines or conduits. The fluid produced from a recovery well is often under conditions of high pressure and low temperature, especially when there is no flow, such as when the well is shut-in. Shut-in pressures may range up to 90 MPa. In deep subsea wells the produced stream is quickly cooled to the subsea temperature which may be as low as (−)1 degree Celsius, is usually around 4 degrees Celsius, and may be above that temperature. Under these conditions gas hydrate crystals can form within the fluids produced from natural gas and crude oil wells and may grow and form deposits within the wellbores of the crude oil or natural gas wells and within the conduits, such as pipelines and other facilities, downstream of the wells. The deposits restrict the flow area and thus the flow rate. In addition, the deposits can break off locally. The broken piece or pieces travel down the flowline and act as a scraper or plow, collecting other deposits. This process leads to an increasing collection of hydrates, which can grow large enough to block or damage the wellbores and other conduits. The problem is especially severe in crude oil and natural gas wells located in places such as the deepwater Gulf of Mexico, where the amount of subcooling typically can reach between 3° Celsius and 30° Celsius. As used herein, the term “subcooling” denotes the difference between the temperature at which the hydrates would decompose at the prevailing pressures, and the environmental temperatures actually present in the wellbore or conduit, if less than the decomposition temperature.
Several methods can be utilized to prevent the blocking of conduits. For example, the conduits may be insulated in an effort to maintain an elevated temperature within the conduit, a temperature above the hydrate formation temperature. This method works while there is flow in the conduit but does not protect against cooling during extended shut-ins caused by storms or other operational needs. Further, insulation is expensive and is difficult to incorporate once a pipeline or other conduit has been built and installed. This is especially true for deepwater crude oil and natural gas wells, where the conduit may be installed under several thousand feet of water.
The shut-in problem can be resolved by heat tracing the conduit either electrically or with a conduit flowing a heated fluid. However, heat tracing further raises the expense of constructing and operating the system.
Melting point depressants (antifreezes), such as the lower alcohols, glycols, and inorganic salts can be used in an attempt to prevent the formation of hydrates. However, at the high subcooling experienced in deep waters, the antifreezes need to be added in substantial amounts, up to quantity equal to the amount of produced water, to be effective.
The use of crystal growth inhibitors or modifiers, as described in U.S. Pat. Nos. 5,460,728, 5,648,575, and 5,879,561, can be utilized to inhibit the formation and/or agglomeration of hydrate crystals. The products used for control of hydrates in the cited patents are ammonium, phosphonium, or sulphonium alkylated compounds, including quaternary compounds. These products are surfactant in that they incorporate parts which are very water soluble (4 to 5 carbon alkyls attached to a charged nitrogen, phosphorous, or sulfur), and parts which prefer to be surrounded by liquid hydrocarbons (at least 8 carbon organic moieties). Overall these compounds, with no more than two 8+ carbon organic moieties, are preferably water soluble, but may also partition into the hydrocarbon phase.
It would be economically advantageous if it were possible to recycle these compounds as a means of alleviating production and disposal costs. U.S. Pat. No. 5,648,575 discloses a method of neutralizing the protonated compound with a base at the end of the conduit or pipeline and recovering the neutralized compound. However, this method results in the breakdown of the surfactant inhibitor compound so as to make recovery of the original compound impossible. In the method of The present invention the compound is recovered intact and ready to be reused over and over again.
The present invention makes the use of This type of hydrate inhibitor/modifier compounds more economical. Finally, by making it possible to recover and reuse the hydrate growth inhibitor/modifier compounds in process fluids, the invention also reduces the costs associated with disposal.